If the system cools normally with air flowing through the radiator (moving along the road) the problem is not coolant.

That doesn't mean that coolant shouldn't be replaced on schedule, but that's more to prevent corrosion as much as it is for actual cooling. If the coolant were inadequate or not flowing properly, it would overheat FASTER while driving along the road (more load, more heat generated) and cool off at idle.

If you want to test this theory, place a large poultry or industrial fan in front of the vehicle and allow it to idle with the hood open and fan running full. It will probably never overheat. However, it will be more effective to simply diagnose the fan clutch to see if it is operating properly - or at all.